Thin gauge steel sheet excellent in surface conditions, formability, and workability and method for producing the same

ABSTRACT

The present invention provides ultralow carbon thin gauge steel sheet and a method for producing the same where coalescence and growth of inclusions in the molten steel are prevented and the inclusions are finely dispersed in the steel sheet, whereby surface defects and cracks at the time of press forming are prevented, growth of recrystallized grains at the time of continuous annealing is promoted, and a high r value (r value≧2.0) and elongation (total elongation≧50%) are exhibited, that is, ultralow carbon thin gauge steel sheet excellent in surface conditions, formability, and workability comprised of, by mass %, 0.0003%≦C≦0.003%, Si &lt;0.01%, Mn ≦0.1%, P≦0.02%, S≦0.01%, 0.0005%≦N≦0.0025%, 0.01%≦acid soluble Ti≦0.07%, acid soluble Al ≦0.003%, and 0.002%≦La+Ce+Nd≦0.02% and a balance of iron and unavoidable impurities, said steel sheet characterized by containing at least cerium oxysulfite, lanthanum oxysulfite, and neodymium oxysulfite.

TECHNICAL FIELD

The present invention relates to ultralow carbon thin gauge steel sheetexcellent in workability and formability, good in surface conditions,and suitable as steel sheet used for press forming for automobiles,household electrical appliances, etc. and a method for producing thesame.

BACKGROUND ART

In general, for automobiles, household electrical appliances, and otherapplications requiring excellent workability, for example, as disclosedin Japanese Patent Publication (B) No. 42-12348 and Japanese PatentPublication (B) No. 54-12883, ultralow carbon steel having a Cconcentration of 0.015 mass % or less and including Ti, Nb, and otherstrong carbide forming elements are being broadly used. Attempts havebeen made to further improve workability up to now by improving themethod of production. Further, Japanese Patent Publication (A) No.3-170618 and Japanese Patent Publication (A) No. 4-52229 propose steelsheet excellent in deep drawability, stretch formability, and otheraspects of workability by increasing the sheet thickness in the finalhot rolling or raising the hot rolled sheet coiling temperature.However, the problem has arisen that the increasing harshness of the hotrolling conditions increases the load on the heating furnace and hotrolling machine.

In the above ultralow carbon steel including Ti or Nb, fine carbides arepresent in the steel, so recrystallization is remarkably suppressed. Forthis reason, annealing at a high temperature becomes necessary. Thereare also issues such as the occurrence of heat buckling or sheetbreakage during rolling and the increase in the amount of energyconsumption. As opposed to this, as shown in Japanese Patent Publication(A) No. 6-212354 and Japanese Patent Publication (A) No. 6-271978, steelsheet with a low recrystallization temperature has been developed bysetting suitable amounts of Mn and P in ultralow carbon steel notcontaining Nb or Ti and changing the hot rolling conditions. However, inthese inventions, Mn or P is added in large amounts, so the alloy costrises and therefore obtaining steel sheet for ultradeep drawing of atotal elongation or 50% or more and and a Lankford value (r value) of2.0 or more is difficult.

Further, ultralow carbon steel sheet usually is produced by deoxidizingby Al not yet deoxidized molten steel decarburized to the ultralowcarbon range in a vacuum degassing system (RH) etc., that is, is “Alkilled steel”, so the molten steel contains a large amount of aluminainclusions. These alumina inclusions easily coalesce and join togetherin the molten steel and remain in the cast slab as large aluminaclusters, so at the time of hot rolling and cold rolling, the aluminaclusters become exposed at the steel sheet surface and cause surfacedefects. Further, when the alumina clusters remain inside the steelsheet, they become the cause of cracks, defects, and other flaws at thetime of press forming. The formability also sharply falls.

In particular, in ultralow carbon steel, if the workability becomesbetter, the susceptibility to surface defects or cracks rises and evenif going to the trouble of developing steel sheet with excellentworkability, the yield obtained as a product is low and a large costincrease is incurred. To deal with these problems accompanying Aldeoxidation, for example, as shown in Japanese Patent Publication (A)No. 61-276756 and Japanese Patent Publication (A) No. 58-185752, themethod has been proposed of treating molten steel by Ca to convert thealumina clusters to low melting point calcium aluminate for quickremoval by floatation. However, conversion of alumina clusters requiresa large amount of Ca. It is known that Ca reacts with the S in the steelto form CaS and becomes a cause of rusting. Further, as shown inJapanese Patent Publication (A) No. 10-226843, the method has also beendeveloped of adding fine amounts of Al and Ti for deoxidation andcontrolling the inclusions in the molten steel to inclusion compositionswith good crushability mainly comprised of Ti oxides, Mn oxides, Sioxides, and alumina.

However, molten steel contains dissolved Al, so if the molten steel isreoxidized by the slag or air, the composition of titania-basedinclusions caused by Ti deoxidation changes to the high alumina side andresults in aggregation and coarsening, so this is not a fundamentalresolution of the problems of surface defects and press defects.Further, the Mn oxides, Si oxides, and Ti oxides have to be madecomplex, but the upper limit value of the amount of addition of Ti islow, so there was the problem that a high workability material could notnecessarily be obtained.

DISCLOSURE OF THE INVENTION

Therefore, the present invention has as its object to solve the aboveproblems all at once and provide an ultralow carbon steel sheet free ofpress cracking and surface deterioration due to inclusions, exhibiting ahigh r value (r value≧2.0) and elongation (total elongation≧50%), andenabling good steelmaking operations and a method for producing thesame.

Specifically, it has as its object to provide an ultralow carbon steelsheet produced not by Al deoxidation, but by Ti deoxidation to preventthe occurrence of the problems due to alumina-based inclusions andAl-based precipitates and by adding a suitable total amount of La, Ce,and Nd to prevent coalescence of titania-based inclusions at the time ofTi deoxidation, control precipitation of Ti-based precipitates, andprevent nozzle clogging in the steelmaking and thereby obtain the aboveproperties.

The present invention was made to solve the above problems and has asits gist the following:

(1) Ultralow carbon thin gauge steel sheet excellent in surfaceconditions, formability, and workability comprised of, by mass %,0.0003%≦C≦0.003%, Si ≦0.01%, Mn≦0.1%, P≦0.02%, S≦0.01%,0.0005%≦N≦0.0025%, 0.01%≦acid soluble Ti≦0.07%, acid soluble Al≦0.003%,and 0.002%≦La+Ce+Nd≦0.02% and a balance of iron and unavoidableimpurities, said steel sheet characterized by containing at least ceriumoxysulfite, lanthanum oxysulfite, and neodymium oxysulfite.

(2) Ultralow carbon thin gauge steel sheet excellent in surfaceconditions, formability, and workability comprised of, by mass %,0.0003%≦C≦0.003%, Si ≦0.01%, Mn≦0.1%, P≦0.02%, S≦0.01%,0.0005%≦N≦0.0025%, 0.01%≦acid soluble Ti≦0.07%, acid soluble Al≦0.003%,and 0.002%≦La+Ce+Nd≦0.02% and a balance of iron and unavoidableimpurities, said steel sheet characterized in that an average grain sizeof recrystallized grains is 15 μm or more and an average value of anaspect ratio of the recrystallized grain size is 2.0 or less.

(3) Ultralow carbon thin gauge steel sheet excellent in surfaceconditions, formability, and workability as set forth in (1) or (2),characterized in that said thin gauge steel sheet further contains, bymass %, 0.0004%≦Nb≦0.05%.

(4) Ultralow carbon thin gauge steel sheet excellent in surfaceconditions, formability, and workability as set forth in any one of (1)to (3), characterized in that said thin gauge steel sheet furthercontains, by mass %, 0.0004%≦B≦0.005%.

(5) A method for producing ultralow carbon thin gauge steel sheetexcellent in surface conditions, formability, and workability comprisingcasting molten steel comprised of, by mass %, 0.0003%≦C≦0.003%,Si≦0.01%, Mn≦0.1%, P≦0.02%, S≦0.01%, 0.0005%≦N≦0.0025%, 0.01%≦acidsoluble Ti≦0.07%, acid soluble Al≦0.003%, and 0.002%≦La+Ce+Nd≦0.02% anda balance of iron and unavoidable impurities, heating the obtained castslab, hot rolling and coiling it to obtain a hot rolled steel strip,cold rolling it by a cold rolling rate of 70% or more, then continuouslyannealing it during which recrystallization annealing it at 600 to 900°C.

(6) A method for producing ultralow carbon thin gauge steel sheetexcellent in surface conditions, formability, and workability as setforth in (5), characterized in that said molten steel further contains,by mass %, 0.0004%≦Nb≦0.05%.

(7) A method for producing ultralow carbon thin gauge steel sheetexcellent in surface conditions, formability, and workability as setforth in (5) or (6) characterized in that said molten steel furthercontains, by mass %, 0.0004%≦B≦0.005%.

BEST MODE FOR WORKING THE INVENTION

Below, the present invention will be explained in detail.

The inventors engaged in detailed research and analysis, taking note ofthe behavior of fine precipitates, on the method of promoting therecrystallization growth at the time of annealing in Ti-containingultralow carbon steel so as to further improve the workability and as aresult discovered that it is effective to limit the dissolved Alconcentration (in analysis, corresponding to the acid soluble Alconcentration, the “acid soluble Al concentration” meaning the measuredamount of Al dissolved in an acid, the fact that dissolved Al willdissolve in an acid, while Al₂O₃ will not dissolve in an acid, beingutilized in this method of analysis) to a predetermined value or lessand to fix the S by at least La, Ce, and Nd. Here, “at least La, Ce, andNd” means one or more types of La, Ce, and Nd.

Steel containing a large amount of dissolved Al produces some fine AlN.This AlN inhibits the recrystallized grain growth at the time ofcontinuous annealing, so by limiting the acid soluble Al concentrationto a predetermined value or less, the precipitation of AlN is prevented.

Further, regarding the S, by adding La, Ce, or CNd into the molten steeland fixing it as relatively large grain size (for example, several μm ormore) lanthanum oxysulfite, lanthanum sulfite, cerium oxysulfite, ceriumsulfite, neodymium oxysulfite, and neodymium sulfite inclusions, thesolute S concentration in the cast slab is reduced. If reducing thesolute S concentration in the cast slab, in the hot rolling process, theS can be prevented from precipitating as fine TiS (diameter of several10 nm) and made to precipitate as the Ti₄C₂S₂ (diameter of several 100nm) larger in grain size than TiS.

Further, before coiling the hot rolled sheet, the C in the steel sheetis also fixed as Ti₄C₂S₂, so the amount of precipitation of finecarbides (diameter of several 10 nm) precipitating at the time ofcoiling can be greatly reduced. That is, by adding at least La, Ce, andNd, it is possible to enlarge the grain size of the precipitates in theTi-containing ultralow carbon steel and possible to reduce the amount ofthe same. The pinning force falls, and the crystal grain growth at thetime of continuous annealing is promoted. As a result, steel sheetexcellent in workability exhibiting a high r value and a high elongationvalue can be obtained.

On the other hand, the inventors studied in detail the behavior ofinclusions in the molten steel of the above composition and, by changingto deoxidation mainly by Ti, succeeded in the fine dispersion ofinclusions and prevention of surface defects, cracks at the time ofpress forming, etc. From the viewpoint of the quality of the material,the acid soluble Al concentration has to be limited to a predeterminedvalue or less and a state where substantively the molten steel does notcontain any dissolved Al has to be secured, so the inventors came upwith the idea of deoxidation by the Ti basically essential for quality.Normally, molten steel decarburized in a converter or vacuum treatmentvessel contains a large amount of dissolved oxygen. This dissolvedoxygen is usually almost entirely removed by the addition of Al (reactsas in the following formula (1)), so a large amount of Al₂O₃ inclusionsare produced.2Al+30=Al₂O₃  (1)

These inclusions coalesce and combine with each other directly afterdeoxidation to form large alumina clusters of several 100 μm or moresize and cause surface defects and cracks at the time of press forming.Further, at the time of continuous casting, these alumina clustersdeposit on the immersion nozzle. In serious cases, the nozzle ends upbeing completely clogged. However, in the present invention, the moltensteel is mainly deoxidized by Ti, so the alumina clusters causingdefects can be kept down to an extremely low limit and, as a result,surface defects and cracks at the time of press forming can be preventedand further clogging of the immersion nozzle can be suppressed. Further,even if slag or air etc. is entrained causing the molten steel toreoxidize, since substantively no dissolved Al is present, no newalumina inclusions are produced.

In the present invention, it is not necessary to remove all of thedissolved oxygen after decarburization by Ti alone. It is also possibleto first perform preliminary deoxidation by Al to an extent where nodissolved Al substantively remains, stir the melt to cause thealumina-based inclusions to float up as coalesced clusters for removalto an extent preventing them from having any effect, then remove theoxygen remaining in the molten steel by Ti. Further, the molten steel ismainly deoxidized by Ti, so the inclusions in the molten steel becomemainly Ti oxides. If continuously casting such molten steel, metalcontaining a high density of Ti oxides deposits on the inside walls ofthe ladle nozzle. In serious cases, the ladle nozzle ends up beingcompletely clogged. The inventors discovered that if adding suitablequantities of La, Ce, and Nd, the Ti-based inclusions in the moltensteel are converted to complex inclusions of at least La oxides, Ceoxides, and Nd oxides with Ti oxides (La oxide-Ti oxide, La oxide-Ceoxide-Ti oxide etc.) and become finely dispersed and form at leastlanthanum oxysulfite, cerium oxysulfite, and neodymium oxysulfite toprevent clogging of the ladle nozzle and that if increasing the amountsof addition of La, Ce, and N, the oxysulfites change to sulfites andconversely clogging of the ladle nozzle is aggravated.

Therefore, by reducing the dissolved Al concentration to below apredetermined value, deoxidizing the molten steel mainly by Ti, andtogether adding suitable quantities of at least La, Ce, and Nd to themolten steel to convert the Ti oxides to complex oxides with the Laoxides, Ce oxides, and Nd oxides and finely disperse them and causingthe formation of at least lanthanum oxysulfite, cerium oxysulfite, andneodymium oxysulfite to fix the solute S, it is possible to prevent theclogging of the immersion nozzle and ladle nozzle and also produce thingauge steel sheet excellent in surface conditions, formability andworkability.

The chemical ingredients of the present invention were limited for thereasons explained below. Note that in the following explanation, theamounts of the ingredients are all mass %.

0.002%≦La+Ce+Nd≦0.02%: The La, Ce, and Nd in steel have the effect ofimproving the workability and of converting and finely dispersing theinclusions. With La+Ce+Nd<0.002%, it is not possible to convert andfinely disperse Ti oxides and, further, it is not possible to fix the Sin the molten steel as oxysulfites. Further, with La+Ce+Nd>0.02%, it ispossible to form sulfites and fix the S, but the ladle nozzle ends upbeing clogged. Therefore, it is necessary to add the La, Ce, and Nd inthe molten steel to obtain 0.002%≦La+Ce+Nd≦0.02%.

Acid soluble Al concentration ≦0.003%: If the acid soluble Alconcentration is high, the recrystallized grain growth at the time ofcontinuous annealing falls and a large amount of alumina clusters isformed in the molten steel causing surface defects and cracks at thetime of press forming, so a level where it is believed there issubstantively no dissolved Al, that is, acid soluble Al concentration≦0.003%, is set. Further, the lower limit value of the acid soluble Alconcentration includes 0%.

0.0003%≦C≦0.003%: If a large amount of C is present in the steel, evenif working the present invention, at the time of coiling, a large amountof fine carbides precipitate and the pinning force increases, so crystalgrain growth is inhibited and the workability ends up falling. For thisreason, it is preferable to reduce the C concentration as much aspossible, but for example if reducing the C concentration to less than0.0003%, the vacuum degasification greatly increases in cost. Therefore,0.003% is aimed at as the upper limit C concentration enabling the rvalue≧2.0 and the total elongation ≧50% of the present invention to beachieved and 0.0003% is aimed at as the lower limit C concentrationbelow which the vacuum degasification greatly increases in cost.

Si≦0.01%: Si is an element useful for raising the strength of the steel,but conversely if the amount added becomes greater, the elongation andother aspects of the workability fall. Therefore, in the presentinvention, total elongation ≧50% was enabled by making the upper limitconcentration of Si 0.01%. The lower limit value of Si concentrationincludes 0%.

Mn≦0.1%: If the Mn concentration becomes high, the workability falls, soto expect a high workability, specifically an r value≧2.0 and a totalelongation≧50%, the upper limit value of the Mn concentration was made0.1%. The lower limit value of Mn concentration includes 0%.

P≦0.02%: If P exceeds 0.02%, the workability is adversely affected andthe r value≧2.0 and total elongation≧50% of the present invention can nolonger be expected, so the upper limit value was made 0.02%. The lowerlimit value of P concentration includes 0%.

S≦0.01%: If S is too great, even if adding Ce or La, the S cannot besufficiently fixed, so fine TiS is precipitated and recrystallized graingrowth is obstructed. For this reason, the upper limit value of S wasmade 0.01%. The lower limit value of S concentration includes 0%.

0.0005≦N≦0.0025%: If N, like C, is present in a solute state, theworkability of the steel sheet is degraded, so the amount is preferablyreduced as much as possible, but for example reducing the Nconcentration to less than 0.0005% would lead to a drop in productivityor a large increase in refining costs, so the lower limit value of N wasmade 0.0005%. Further, if the N concentration is high, a large amount ofTi has to be added. Along with this, fine TiS ends up precipitatingregardless of the addition of La or Ce, so the upper limit value of Nwas made 0.0025%. 0.01%≦acid soluble Ti≦0.07%: Ti is one of the mostimportant elements in the present invention. Ti has to be added in anamount required for deoxidation of the molten steel and an amount formaintaining the above range of acid soluble Ti. Ti is added for thepurpose of fixing the C and N degrading the workability and deoxidizingthe molten steel, so must be present in the molten steel as dissolved Ti(in analysis, corresponding to the acid soluble Ti concentration, the“acid soluble Ti concentration” meaning the measured amount of Ti solutein an acid, the fact that dissolved Ti will dissolve in an acid, whileTi₂O₃ will not dissolve in an acid, being utilized in this method ofanalysis). If the acid soluble Ti concentration exceeds 0.07%, even ifLa, Ce is added, fine TiS ends up precipitating, while if the acidsoluble Ti concentration becomes lower than 0.01%, the C and N in thesteel sheet cannot be sufficiently fixed and the dissolved oxygen in themolten steel will also not fall, so the Ti concentration was made0.01%≦acid soluble Ti<0.07%.

0.004%≦Nb≦0.05%: Nb improves the workability, so is added to fix the Cand N. If the amount of addition is less than 0.004%, the effect ofimproving the workability becomes smaller, while if the amount ofaddition is over 0.05%, the presence of the solute Nb conversely causesthe workability to easily deteriorate, so the Nb concentration ispreferably made 0.004%≦Nb≦0.05%.

0.0004%≦B≦0.005%: B is an element effective for preventing theembrittlement called “secondary work embrittlement” often seen whenthere is no longer solute C present at the crystal grain boundaries. Itis added when the steel sheet of the present invention is used for partswhich are subjected to extreme drawing etc. If the amount of addition isless than 0.0004%, the effect of prevention of secondary workembrittlement becomes smaller, while if over 0.005%, therecrystallization temperature becomes higher and other trouble easilyoccurs, so the amount of addition of B is preferably made0.0004%≦B≦0.005%.

Next, the reasons for limitation of the production conditions will beexplained. The continuously cast slab obtained from the aboveingredients may be cooled once, reheated, then hot rolled or may bedirectly hot rolled directly without cooling. The temperature of the hotrolling, to cause as much Ti₄C₂S₂ as possible to precipitate, should benot more than 1250° C., preferably not more than 1200° C. In the presentinvention, C ends up precipitating almost entirely before coiling of thehot rolled sheet, so the coiling temperature has no effect on the amountof precipitation of fine carbides. The sheet should be coiled at usuallyfrom room temperature to about 800° C. in range. Coiling.at less thanroom temperature not only results in excessive facilities, but also doesnot give any particular effect of improvement. Further, if the coilingtemperature exceeds 800° C., the oxide scale becomes thicker and invitesan increase in the cost of pickling.

Next, the reduction rate in the cold rolling (called the “cold rollingrate”) has to be at least 70% from the viewpoint of securing theworkability. If the cold rolling rate is less than 70%, an r value of2.0 or more cannot be secured.

The cold rolled steel sheet obtained after the cold rolling process iscontinuously annealed. The continuous annealing is performed at atemperature of 600 to 900° C. If less than 600° C., the steel does notrecrystallize and the workability deteriorates, so 600° C. is made thelower limit, while if over 900° C., the steel sheet weakens in hightemperature strength and problems arise such as the sheet breaking inthe continuous annealing furnace, so 900° C. is made the upper limit.After this, skin pass rolling may be performed. Further, after this, thesheet may also be plated for corrosion resistance. The continuousannealing may be performed at the hot dip zinc coating line. It is alsopossible to hot dip coat the sheet immediately after annealing to obtaina hot dip zinc coated steel sheet, alloyed hot dip zinc coated steelsheet, etc.

The inventors investigated the recrystallized grains of the thusobtained high workability steel sheet in detail, whereupon they found itis possible to obtain steel sheet having an average circle equivalentdiameter of recrystallized grains of 15 μm or more and an average valueof the long axis/short axis of recrystallized grains (aspect ratio) of2.0 or less. This is because the fine precipitates are reduced in numberand the growth of the recrystallized grains is promoted.

When the average circle equivalent diameter of the recrystallized grainsof the steel sheet is 15 μm or more, the total elongation is improved to50% or more. The upper limit is not particularly defined.

Further, when the average value of the long axis/short axis ofrecrystallized grains (aspect ratio) is 2.0 or less, the recrystallizedgrains approach spherical shapes and the r value is improved to 2.0 ormore. Further, the lower limit value is not particularly defined, butthe closer the crystallized grains to a spherical shape, the smaller theanisotropy, so the aspect ratio is preferably as close to 1 as possible.

EXAMPLES

Molten steel right after discharge from the converter was decarburizedby a vacuum degasification system, then predetermined ingredients wereadded to thereby produce molten steel comprised of each of theingredient compositions of Table 1. Each molten steel was continuouslycast to obtain a cast slab which was heated to 1150° C., finish hotrolled at 930° C., and coiled at 700° C. to obtain a hot rolled sheet ofa thickness of 4 mm. The obtained hot rolled sheet was cooled by areduction rate of 80% (reduction rate=(initial sheet thickness−finalsheet thickness)/initial sheet thickness×100), then continuouslyannealed at 780° C. and further skin pass rolled at a reduction rate of0.7% to obtain the final product sheet. The obtained final product sheetwas subjected to a tensile test and measured for r value using a No. 5test piece described in JIS Z2201. The r value was calculated bymeasuring the values in the rolling direction (L direction), a directionperpendicular to the rolling direction (C direction), and a directioninclined 45° with respect to the rolling direction (D direction) andobtaining the average by the following equation:r=(r _(L)+2r _(D) +r _(c))/4

Each final product sheet was polished at the cross-section perpendicularto the rolling direction and examined for inclusions by the secondaryelectron image of a scan type electron microscope. EDX was used foranalysis of the composition of about 50 randomly selected inclusions soas to determine the main inclusion composition. Further, the finalproduct sheet was measured for the average circle equivalent diameterand average aspect ratio of the recrystallized grains by using a nitalreagent to corrode the cross-section of the steel sheet in the rollingdirection, obtaining a 500+ to 1000+ optical micrograph, then analyzingthe image. The quality was evaluated by visual observation on theinspection line after cold rolling and assessing the number of surfacedefects occurring per coil.

The results of evaluation of the thus obtained steel sheets are shown inTable 2. As clear from Table 2, the steel sheets of the inventionexamples satisfying the requirements of the present invention (SteelNos. 1 to 5) are steel sheets which have the solute S fixed as at leastlanthanum oxysulfite, cerium oxysulfite, and neodymium sulfiteinclusions, have average recrystallized grain sizes of 15 μm or more andaspect ratios of 2.0 or less, and are extremely good in grain growth, soexhibit high r values (r value≧2.0) and good total elongations (totalelongation ≧50%) and are improved in workability. Further, it is learnedthat the surface conditions are also extremely good in the inventionexamples (Steel Nos. 1 to 5) since almost no surface defects are formed.Further, in the invention examples (Steel Nos. 1 to 5), the Ti oxides inthe molten steel are converted to complex oxides of at least La, Ce, andNd oxides with Ti oxides, so there is also no clogging of the ladlenozzle or immersion nozzle and the operability at the time of continuouscasting is also extremely good.

As opposed to this, in the steel sheets of the comparative examples(Steel Nos. 6 to 10), since La, Ce, and Nd are not added, no lanthanumoxysulfite, cerium oxysulfite, and neodymium sulfite inclusions areformed at all, a large amount of solute S remains, and steel sheetshaving average recrystallized grain sizes of less than 15 μm and aspectratios of over 2.0 and poor in grain growth are obtained, so the rvalues (r value<2.0) and total elongations (total elongation <50%) arelow and the workabilities are not improved. Further, regarding thesurface conditions as well, in the comparative examples (Steel Nos. 6 to9), since the inclusions are alumina, surface defects are formed.Further, in the comparative examples (Steel Nos. 6 to 9), the alumina inthe molten steel deposits on the immersion nozzle and nozzle cloggingoccurs. In one comparative example (Steel No. 10), Ti oxides depositedon the ladle nozzle and the casting was interrupted. TABLE 1 Ingredients(mass %) Acid Acid Steel soluble soluble no. C Si Mn P S N Nb B Ti AlLa + Ce + Nd Remarks 1 0.0025 0.008 0.09 0.018 0.009 0.0018 0.03 0.0010.0025 Inv. ex. 2 0.0028 0.005 0.08 0.017 0.007 0.0024 0.012 0.05 0.00150.01 Inv. ex. 3 0.0018 0.009 0.05 0.012 0.005 0.002 0.0003 0.015 0.00250.018 Inv. ex. 4 0.0008 0.004 0.07 0.014 0.008 0.0022 0.025 0.0003 0.060.002 0.008 Inv. ex. 5 0.0012 0.003 0.05 0.008 0.009 0.0015 0.02 0.0010.005 Inv. ex. 6 0.0025 0.008 0.09 0.018 0.009 0.0018 0.03 0.038 trComp. ex. 7 0.0028 0.005 0.08 0.017 0.007 0.0024 0.012 0.05 0.04 trComp. ex. 8 0.0018 0.009 0.05 0.012 0.005 0.002 0.0003 0.015 0.035 trComp. ex. 9 0.0008 0.004 0.07 0.014 0.008 0.0022 0.025 0.0003 0.06 0.04tr Comp. ex. 10 0.0012 0.003 0.05 0.008 0.009 0.0015 0.02 0.001 tr Comp.ex.

TABLE 2 Average No. of Total recrystallized Average surface Steel relongation grain size aspect defects no. value (%) (mm) ratio Inclusioncomposition (/coil) Remarks 1 2.2 52 19 1.7 Complex inclusions of La, 0Inv. ex. Ce, Nd oxides and Ti oxides Oxysulfites of La, Ce, and Nd 2 2.151 17 1.9 Complex inclusions of La, 0 Inv. ex. Ce, Nd oxides and Tioxides Oxysulfites of La, Ce, and Nd 3 2.4 54 20 1.6 Complex inclusionsof La, 0 Inv. ex. Ce, Nd oxides and Ti oxides Oxysulfites of La, Ce, andNd 4 2.5 56 22 1.4 Complex inclusions of La, 0 Inv. ex. Ce, Nd oxidesand Ti oxides Oxysulfites of La, Ce, and Nd 5 2.4 55 21 1.5 Complexinclusions of La, 0 Inv. ex. Ce, Nd oxides and Ti oxides Oxysulfites ofLa, Ce, and Nd 6 1.7 45 9 2.4 Alumina-based inclusions 5.2 Comp. ex. 71.6 44 8 2.5 Alumina-based inclusions 7.3 Comp. ex. 8 1.8 46 10 2.3Alumina-based inclusions 6.2 Comp. ex. 9 1.9 48 14 2.1 Alumina-basedinclusions 5.6 Comp. ex. 10 1.8 47 13 2.1 Ti oxide-based inclusions 0Comp. ex.

INDUSTRIAL APPLICABILITY

According to the present invention, the inclusions in the molten steelcan be finely dispersed, so clogging of the immersion nozzle and ladlenozzle is suppressed, surface defects and cracks at the time of pressforming can be prevented, and recrystallized grain growth at the time ofcontinuous annealing can also be promoted, so low carbon thin gaugesteel sheet excellent in workability and formability can be produced.

1. Ultralow carbon thin gauge steel sheet excellent in surfaceconditions, formability, and workability comprised of, by mass %,0.0003%≦C≦0.003%, Si≦0.01%, Mn ≦0.1%, P≦0.02%, S≦0.01%,0.0005%≦N≦0.0025%, 0.01%≦acid soluble Ti≦0.07%, acid soluble Al≦0.003%,and 0.002%≦La+Ce+Nd≦0.02% and a balance of iron and unavoidableimpurities, said steel sheet characterized by containing at least ceriumoxysulfite, lanthanum oxysulfite, and neodymium oxysulfite.
 2. Ultralowcarbon thin gauge steel sheet excellent in surface conditions,formability, and workability comprised of, by mass %, 0.0003%≦C≦0.003%,Si≦0.01%, Mn ≦0.1%, P≦0.02%, S≦0.01%, 0.0005%≦N≦0.0025%, 0.01%≦acidsoluble Ti≦0.07%, acid soluble Al≦0.003%, and 0.002%≦La+Ce+Nd≦0.02% anda balance of iron and unavoidable impurities, said steel sheetcharacterized in that an average grain size of recrystallized grains is15 μm or more and an average value of an aspect ratio of therecrystallized grain size is 2.0 or less.
 3. Ultralow carbon thin gaugesteel sheet excellent in surface conditions, formability, andworkability as set forth in claim 1, characterized in that said thingauge steel sheet further contains, by mass %, 0.0004%≦Nb≦0.05%. 4.Ultralow carbon thin gauge steel sheet excellent in surface conditions,formability, and workability as set forth in claims 1, characterized inthat said thin gauge steel sheet further contains, by mass %,0.0004%≦B≦0.005%.
 5. A method for producing ultralow carbon thin gaugesteel sheet excellent in surface conditions, formability, andworkability comprising casting molten steel comprised of, by mass %,0.0003%≦C≦0.003%, Si≦0.01%, Mn≦0. 1%, P≦0.02%, S≦0.01%,0.0005%≦N≦0.0025%, 0.01%≦acid soluble Ti≦0.07%, acid soluble Al≦0.003%,and 0.002%≦La+Ce+Nd≦0.02% and a balance of iron and unavoidableimpurities, heating the obtained cast slab, hot rolling and coiling itto obtain a hot rolled steel strip, cold rolling it by a cold rollingrate of 70% or more, then continuously annealing it during whichrecrystallization annealing it at 600 to 900° C.
 6. A method forproducing ultralow carbon thin gauge steel sheet excellent in surfaceconditions, formability, and workability as set forth in claim 5,characterized in that said molten steel further contains, by mass %,0.0004%≦Nb≦0.05%.
 7. A method for producing ultralow carbon thin gaugesteel sheet excellent in surface conditions, formability, andworkability as set forth in claim 5 characterized in that said moltensteel further contains, by mass %, 0.0004%≦B≦0.005%.